1. Field of the Invention
The present invention relates to an inkjet printhead, and more particularly, to a microinjector of the ink jet printhead.
2. Description of the Prior Art
In recent years, a microinjector for ejecting fluids, such as gas, ink, chemical solutions and other liquid materials, has been widely applied to fluid-ejecting apparatuses like an inkjet printhead in an inkjet printer. As microinjectors become cheaper and more reliable, and as high quality fluids of high frequency and spatial resolution come to the market, the microinjector is becoming more and more popular and has a wide number of uses. For example, a microinjector can be applied to a variety of industrial fields, such as a fuel injection system, a cell sorting system, a drug delivery system, a micro jet propulsion system, and print lithography.
Please refer to FIG. 1, which is a schematic diagram of a microinjector 10 according to the prior art. The microinjector 10 is disclosed in a U.S. Pat. No. 6,102,530, “Apparatus and method for using bubbles as virtual value in microinjector to eject fluid”. The microinjector 10 comprises a chamber 12, a manifold 14 connected to the chamber 12, an orifice 16 disposed above the chamber 12, a first heater 18, a second heater 20, and a SiO2 layer (not shown). The first and second heaters 18 and 20 are both disposed proximately adjacent to the orifice 16 and external to the chamber 12. The first and second heaters 18 and 20 are typically electrodes connected in series to a common electrode (not shown). The chamber 12 and the manifold 14 of the microinjector 10 are filled with fluid (not shown).
The first heater 18 has a cross sectional area smaller than that of the second heater 20, and the first heater 18 accordingly has a heat efficiency higher than that of the second heater 20. Therefore, driven by the same common electrode, the first heater 18 generates a first bubble 22 earlier than the second heater 20 generates a second bubble 24. It can be seen that the first bubble 22 has a volume bigger than that of the second bubble 24.
The first heater 18 generates the first bubble 22 that is big enough to form a virtual valve to prevent the fluid contained in the manifold 14 from entering the chamber 12 in order to diminish a cross talk effect between the chamber 12 and other chambers neighboring the chamber 12 to impact the chamber 12 of the microinjector 10. At the same time, the second bubble 24, with a growing volume driven by the second heater 20, ejects the fluid confined in the chamber 12 through the orifice 16 to a region outside of the chamber 12 gradually.
Please refer to FIG. 2, which is another schematic diagram of the microinjector 10 according to the prior art. As the second bubble 24 grows and has a volume large enough to contact with the first bubble 22, the first bubble 22 combined with the second bubble 24 are capable of preventing fluid confined in a region 26 opposite to the orifice 16 from being ejected to a region outside of the chamber 12, omitting the satellite droplets.
After the fluid has been ejected to a region outside of the chamber 12 by the combination of the first and second bubbles 22 and 24, the common electrode stops driving the first and second heaters 18 and 20. Therefore, the volumes of the first and the second bubbles 22 and 24 decrease gradually and the chamber 12 is filled with fluid again.
Please refer to FIG. 3, which is a schematic diagram of a silicon wafer 30 ready to be etched into the microinjector 10 according to the prior art. The silicon wafer 30 comprises a phosphosilicate-glass (PSG) 32 as a sacrificial layer and a low stress silicon nitride 34 as a top surface of the chamber 12. In a bulk etching process for the silicon wafer 30, the silicon wafer 30 is etched in a solution of potassium hydroxide (KOH), while the sacrificial layer 32 of the silicon wafer 30 is removed by hydroflouric acid (HF). Experiments show that the chambers 12 top surface 34, which is formed by the low stress silicon nitride, is fragile and easily cracked. The KOH solution probably etches a surface of the silicon wafer 30 and therefore reduces a yield rate of the silicon wafer 30 or even damages the silicon wafer 30.
The experiments also show that the silicon nitride 34, further coated with a layer of metal, such as gold and nickel, not only has a more rigid structure, the silicon nitride 34 also has an additional radiation function, for smoothing the fabrication of the manifold 14 and the orifice 16.
In the microinjector 10, the SiO2 layer has a dielectric constant of approximately 3.9–4.5, and a thickness of 0.5 μm, while the silicon nitride 34 has a dielectric constant of approximately 6–8, and a thickness of 0.5 μm. The metal layer, which is coated on the silicon nitride 34, has a large area and a corresponding large parasitic capacitance. Such a large parasitic capacitance results in the metal layer accumulating a great deal of charge. This charge easily couples to circuits disposed on a region under the silicon nitride 34, deforming a square wave driving the microinjector 10 to have a shark-fin-shaped overshoot waveform, as shown in FIG. 4. The square wave with the overshoot probably damages the first and second heaters 18 and 20 sequentially or a MOS transistor in the microinjector 10. Moreover, too large of a parasitic capacitance also accompanies an increasing RC, thereby reducing driving frequency as well as printing efficiency, creating a problem of signal delay.